This invention relates in general to electric glow discharge devices and methods for treating metallic tubes and in particular to a new and useful device particularly for treating the inside of a tube which includes a vacuum chamber having a cathode mounted within the tubular article to be treated in a vacuum chamber so that the cathode and the article are rotatable relative to each other and to an improved method of treating the article.
Such an apparatus for treating the interior of a tube using an electric glow discharge is known from German Pat. No. 976,529. This prior art apparatus is not suitable for depositing by cathode sputtering on the inside surface of a tube a sufficiently adherent and dense layer of electrode material, since the atoms knocked out of the auxiliary electrode lose a major portion of their kinetic energy by colliding with the gas atoms and molecules prior to reaching the surface to be coated. This energy, however, would be well utilized upon impinging on the inside of the tube, for displacing or dislodging the gas atoms or molecules there sorbed, and thus for building up a firmly adhering and dense layer. An apparatus of this kind, as far as usable for producing high quality layers, has substantial advantages over other prior art devices for the inside coating of tubes, such as described, for example in the following disclosures: German No. AS 2,820,301 German No. OS 2,544,942; German No. OS 2,729,286; Publication J. L. Vossen and W. Kern, "Thin Film Processes", Academic Press, 1978.
Devices of this kind require magnetic fields or high-frequency alternating fields and are therefore expensive and complicated, in both manufacture and operation. Greatest difficulties arise if ferromagnetic tubes are to be coated, since in such an event, magnetic fields are varied in a manner suppressing an effective gas ionization. Another problem arises while attempting reactive sputtering of the auxiliary electrode introduced into the tube. Since in this case, the reactive gas is incorporated into the connecting layer, thus consumed, the reactive gas component in the central zone between the open tube ends is depleted, particularly in instances where the sputtering equipment occupies most of the space inside the tube to be coated. A gas distributor such as disclosed in British Pat. No. 1,570,044 cannot be employed for an inside coating of narrow tubes.